Jeannine Stein, a California native, wrote about health for the Los Angeles Times. In her pursuit of a healthy lifestyle she has taken countless fitness classes, hiked in Nepal and got in a boxing ring.

Teens who start smoking marijuana as teenagers and use it persistently could lose a substantial amount of IQ points—and they may never get them all back.

What happens to the adolescent brain when pot is an ongoing part of the equation was the subject of a study released this week in the Proceedings of the National Academy of Sciences. In it, researchers tracked 1,037 New Zealanders who were part of a longitudinal study.

The group was given psychological tests at age 13, before marijuana smoking started, then again at age 38, after patterns of using—or not using—pot were set. The tests measured a number of mental functions, such as intelligence, memory, reasoning, visual processing and processing speed.

Researchers also wanted to see if a number of variables had any sway over the results: Could mental decline be blamed on alcohol or drug use, schizophrenia, or less education? Are teen pot smokers' brains particularly vulnerable to the drug? Were cognitive damages noticed across the board, or were certain mental functions affected more than others?

Study participants who started smoking pot as teens and used pot persistently showed a drop in IQ points. Those who never smoked a joint actually showed a slight increase in IQ, while those who started smoking as teens and continued to smoke in subsequent years dropped an average of eight points over the course of the study.

About 5 percent of the study group was dependent on marijuana, defined as using it more than once a week, before age 18.

The cognitive effects were seen across different areas of brain function, and daily function was affected as well. Education levels didn’t seem to influence the results.

For those who smoked pot persistently and quit, cognitive function didn’t appear to improve a year after quitting. People who became continual pot users as adults, not teens, didn’t show a drop in IQ.

The findings, the authors said, seem to be in line with the theory that smoking pot at a young age, when the brain is still in critical stages of development, may cause damage to the brain. Missing from the data, however, is the why, the mechanism linking pot use to cognitive decline.

Even without that, lead author Madeline Meier of Duke University warned of pot’s potential destructiveness: “Marijuana is not harmless, particularly for adolescents,” she said in in a news release.

Meier told CBS News, “Parents should understand that their adolescents are particularly vulnerable.”

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